1. Field of the Invention
The present invention relates generally to a steering force control system, for a power steering system of a vehicle, which includes two reaction force chambers and provides a steering force controlled in accordance with the speed of the vehicle and particularly to a steering force control system in which an inlet fluid pressure from a pump is regulated to a steering pressure either stepwise or continuously varied in accordance with the vehicle speed provides a reaction force for a control valve moved for admitting the steering pressure into one fluid chamber of a power cylinder and draining the other fluid chamber of the power cylinder which force is increased with increases in the vehicle speed.
2. Description of the Prior Art
As is well known in the art, when a power steering system for a vehicle is a so-called integral type which is mechanically connected to a load through a torsion bar at all times, a steering force can be increased in accordance with increases in the speed of the vehicle by, for example, reducing the quantity of pressurized hydraulic fluid discharged from a pump or by controlling a steering pressure by a relief valve. However, when a power steering system for a vehicle is a linkage type which includes a reaction force mechanism, since a reaction force is produced by a steering pressure, simply reducing the steering pressure has been attended with inconveniences that a steering pressure for a power cylinder and the reaction force are reduced. Furthermore, when a power steering system is a linkage type which includes one reaction force chamber, a steering force can be controlled by controlling the pressure of hydraulic fluid in the reaction force chamber in accordance with the vehicle speed. However, when a power steering system is a linkage type which includes two reaction force chambers, a steering force has been unable to be controlled in accordance with the vehicle speed. In this instance, it is necessary to control the pressure of hydraulic fluid in the reaction force chambers in accordance with both the vehicle speed and the steering operations.